Ethylene carbonate (EC) and dimethyl carbonate (DMC) are important chemical substances of the lithium-ion battery (LIB) electrolytes. Although the carbonate esters are regarded as a cause of the LIB fires, there are few studies on the pyrolysis/combustion characteristics of EC. This study aims to investigate the pyrolysis characteristics of EC by performing theoretical calculations and species measurements, and to propose the first EC pyrolysis sub-mechanism. The theoretical calculations performed for a gas-phase EC decomposition at the G4 level of theory indicate that a dissociation channel producing acetaldehyde and carbon dioxide, EC = CH3CHO + CO2 (R1), is an energetically and entropically preferred decomposition reaction channel. The rate constant of R1 calculated in the present study is 7.4 times higher than that of the CO2 elimination reaction of DMC, which produces dimethyl ether, DMC = DME + CO2, at 1000 K. Species measurements were performed for the pyrolysis of EC/DMC/N2 mixtures at atmospheric pressure using a time-of-flight mass spectrometer and a gas chromatograph connected to a micro flow reactor with a controlled temperature profile. Oxygenates (EC, DMC, formaldehyde, acetaldehyde and DME), inorganic species (H2, CO and CO2) and hydrocarbons (CH4, C2H2, C2H4 and C2H6) were measured. Experimental results indicate that the consumption of EC occurs at a lower temperature range (Tw = 1000–1050 K) than that of DMC (Tw = 1050–1100 K), as was expected from the theoretical calculations. The measured acetaldehyde showed a rapid increase at Tw = 1000–1050 K, implying that acetaldehyde can be a good marker for the EC decomposition. One-dimensional computations using the present model involving the EC pyrolysis sub-mechanism reproduced the measured results well. Based on the reaction path analysis, R1 accounts for approximately 95% of the contribution to EC consumption at 1080 K, where 30% of EC is consumed.